High resolution, room temperature spectroscopy of gamma rays requires semiconductor gamma detectors such as CdZnTe or TlBr. In order to achieve the best performance, these detectors utilize advanced electrical contact and readout schemes including pixilation, co-planar grids, and hemispherical contacts.
These schemes require an electric field both through the bulk of the device and often across a given surface. This results in electronic noise from leakage current, which can be defined as either bulk or surface leakage current. Electronic noise arises from current injected into a CdZnTe (CZT) detector that flows along the surface and/or through the bulk thereof; current generated by defects along the surface of the CZT, bursts of anomalous noise, and/or buildup of charge at non-Ohmic contacts.
The current state of the art for CdZnTe gamma detectors uses contacts deposited on an as-polished surface, such as the detector 100 shown in FIG. 1. The detector 100 includes a bulk portion 102 composed of a suitable detector material for the particular target to be detected (e.g. cadmium-zinc-tellurium (CdZnTe, or CZT) for detecting gamma radiation). The bulk portion 102 has formed on a surface thereof metal contacts 104, comprising a material suitable for use as an electrode, e.g. gold or aluminum (which act as Ohmic or Schottky contacts), and the contacts 104 may optionally be formed on doped portions 106, 108 of the surface of the bulk portion 102. Ohmic contacts typically display higher leakage current while Schottky contacts may display charge buildup at the interface, distorting the electric field. The optional doped portions 106, 108, which are notably excluded from most conventional CZT detector structures, may be doped with a suitable dopant species such as aluminum or phosphorous. Alternatively, a layer of amorphous silicon or selenium (not shown) may be formed in place of the doped portions 106, 108, and may extend across the entire width of the structure as shown in FIG. 1.
Notably, the doped portions 106, 108 are not in physical contact, nor are they electrically coupled/contacting. A doped layer 110 is formed, again optionally, on a surface of the bulk portion 102 opposite the surface onto which contacts 104 are formed; and a final Ohmic contact layer 112 is formed on a surface of the doped layer 110 opposite the bulk portion 102. One or more surfaces of the bulk portion 102 and/or doped portions/layer 106, 108, 110 may be treated via etching and oxidation to generate passivating layers 114 on surface(s) of the corresponding portions, as shown in FIG. 1.
Conventional detector configurations such as shown in FIG. 1 can achieve low leakage current but still suffer from injected current at the contacts which acts as a source of electronic noise. In addition to the leakage current, anomalous bursting noise is commonly observed at high biasing fields. Finally, buildup of charge at non-Ohmic contacts can distort the applied electric field. Because of this, detectors are operated at lower fields than may be optimal.
Accordingly, it would be useful to provide systems and techniques that minimize or eliminate injected current and defect generated noise, enabling operation of detectors at higher field strengths to improve signal collection ability.